Thermoformable barrier films

ABSTRACT

The invention relates to a thermoformable multilayer foil encompassing at least one polyolefin layer based on a thermoplastic polyolefin, olefin copolymer, or mixture of these, and at least one gas-barrier layer, where the oxygen transmission of the multilayer foil is at most 0.5 cm 3  m −2  d −1  bar −1 .

The invention relates to a thermoformable multilayer foil which features particular oxygen- and water-vapor-barrier properties. The inventive multilayer foil is particularly suitable for use in self-cooling transport containers.

The transport of goods whose sensitivity to heat requires that they be stored at a temperature below ambient temperature is commercially significant. A wide variety of goods require continuous and uninterrupted cooling during transport. Examples which may be mentioned are foodstuffs, fine chemicals, biochemical or molecular-biology products, and also organs.

Containers used for transport of goods of this type can be classified on the one hand with respect to the temperature range maintained therein or the predicted transport time, and on the other hand with respect to size. Containers whose capacity is some m³ are usually cooled with the aid of electrically operated cooling equipment. Smaller containers, i.e. those whose capacity is a few liters, are cooled with the aid of dry ice or gels, for example.

Cooling systems developed relatively recently as alternatives to dry ice and gels are based on evaporation of a suitable liquid at reduced pressure.

The mode of operation of cooling systems of this type is based on the principle that heat (enthalpy of evaporation) is withdrawn from the environment via the evaporation of the liquid, and this phenomenon can be utilized for cooling. The boiling point of a liquid can be lowered via pressure reduction. If the pressure is sufficiently low, the liquid boils and thus absorbs heat from the environment. In order that the boiling process proceeds continuously, there must be no saturation of vapor above the liquid. A requirement is therefore that the evaporated liquid be continuously withdrawn from the system, without penetration of external air into the system. If the liquid is water, the water vapor can, for example, be removed from the system with the aid of a hygroscopic substance. In this connection reference may be made to U.S. Pat. No. 6,584,797, U.S. Pat. No. 6,688,132, and U.S. Pat. No. 6,701,724, for example.

In order that cooling systems of this type function satisfactorily, it is desirable that the subatmospheric pressure be about 5×10⁻³ bar or less. This subatmospheric pressure should be maintained as required by the cooling system for an appropriate storage time, for example for two years.

U.S. Pat. No. 6,584,797 and U.S. Pat. No. 6,688,132 propose surrounding the cooling system with a metallized polyester foil and evacuating the resultant interior space. U.S. Pat. No. 6,701,724 mentions, as an alternative, introducing the cooling system into a thermoformed tray composed of a semirigid plastic, and using a metallized lid foil for sealing, thus producing an enclosed space within which the subatmospheric pressure is generated and maintained.

Materials suitable for production of this type of thermoformed tray firstly require sufficient impermeability to gases and to water vapor and secondly require sufficient thermoformability. However, it is difficult to combine these properties successfully with one another.

For example, the materials of the prior art whose impermeability to gases and to water vapor would be sufficient to maintain this type of subatmospheric pressure for a prolonged period usually have one or more metallized layers, which are responsible for the high level of barrier action (cf., for example, DE-A 10025305 and DE-A 10047043). However, metallized multilayer foils have the disadvantage of not being thermoformable, since the metal film, the layer thicknesses of which are usually only a few nm, fractures as a result of the thermoforming process and the barrier action is thus lost.

In contrast, conventional thermoformable multilayer foils usually do not exhibit the required barrier action, and the vacuum would therefore not be maintained for a sufficient period.

There is therefore a need for thermoformable multilayer foils with a high level of barrier action, where the barrier action should be substantially maintained even after the thermoforming process.

An object on which the invention is based is therefore to provide multilayer foils which have advantages over the multilayer foils of the prior art. The multilayer foils should firstly feature good thermoformability, in particular deep-draw thermoformability on conventional machines, and secondly should feature high impermeability to gases and to water vapor, thus making them suitable for production of thermoformed trays for the cooling systems described above.

This object is achieved via the subject matter of the claims. Surprisingly, it has been found possible to produce thermoformable multilayer foils whose oxygen transmission prior to and after the thermoforming process is at most 0.5 cm³ m⁻² d⁻¹ bar⁻¹ (at 23° C. and 75% rel. humidity).

The invention provides a thermoformable multilayer foil encompassing at least one polyolefin layer based on a thermoplastic polyolefin, olefin copolymer, or mixture of these, and at least one gas-barrier layer, where the oxygen transmission of the multilayer foil is at most 0.5 cm³ m⁻² d⁻¹ bar⁻¹ (at 23° C. and 75% rel. humidity).

The inventive multilayer foil is thermoformable, preferably deep-draw thermoformable. For the purposes of the invention, the expression “deep-draw thermoformable” defines a material which can be “deep-draw thermoformed” when exposed to heat in a suitable device, i.e. by way of example can be formed to give a packaging tray on exposure to pressure (and/or vacuum). This is a material which has thermoplastic properties, and is therefore deformable when heated, but which has sufficient dimensional stability at room temperature so that the shape (e.g. packaging tray) prescribed via deep-draw thermoforming is retained even after introduction of the goods for packaging. In order that the inventive multilayer foil is thermoformable, it is unmetallized at least in the regions intended to be thermoformed, and moreover these regions have no coating with an inorganic oxide, e.g. Al_(x)O_(y) or SiO_(x).

The inventive multilayer foil preferably encompasses n gas-barrier layers and n+1 polyolefin layers, where n can be 1, 2, 3, 4, or 5. In principle, n can also be greater than 5, but multilayer foils of that type are less preferred according to the invention.

It is preferable that the inventive multilayer foil encompasses

-   -   n gas-barrier layers B_(i), where i=from 1 to n; and     -   n+1 polyolefin layers P_(i), where i from 1 to n+1;         where     -   n=1, 2, 3, 4, or 5;     -   the polyolefin layers, respectively identical or different, are         based on a thermoplastic polyolefin, olefin copolymer, or         mixture of these; and     -   for a given i, the gas-barrier layer B_(i) has been arranged         between the polyolefin layers P_(i) and P_(i+1).

If n=1, the inventive multilayer foil encompasses a gas-barrier layer B₁ and two polyolefin layers P₁ and P₂, where the gas-barrier layer B₁ has been arranged between the two polyolefin layers P₁ and P₂ (P₁/B₁/P₂). If n=2, the inventive multilayer foil encompasses two gas-barrier layers B₁ and B₂ and three polyolefin layers P₁, P₂, and P₃, where the gas-barrier layer B₁ has been arranged between the two polyolefin layers P₁ and P₂ and the gas-barrier layer B₂ has been arranged between the two polyolefin layers P₂ and P₃ (P₁/B₁/P₂/B₂/P₃). For n=3, the corresponding result is the layer sequence P₁/B₁/P₂/B₂/P₃/B₃/P₄, and for n=4, the layer sequence P₁/B₁/P₂/B₂/P₃/B₃/P₄/B₄/P₅.

For the purposes of the description, two successive layers within the layer sequence are separated from one another via “/”, but the layers here do not necessarily directly adjoin one another, i.e. have contact with one another—it is also possible that one or more layers have been inserted between them. The expression “P₁/B₁/P₂/B₂/P₃” therefore also encompasses by way of example a P₁/B₁/P₂/X/Y/B₂/P₃ multilayer foil, where “X” and “Y” are other layers, for example two additional layers, identical or different, based on a polyolefin or olefin copolymer.

The polyolefin layers of the inventive multilayer foil, identical or different, are based on a thermoplastic polyolefin, olefin copolymer or mixture of these.

For the purposes of the description, polyolefins and olefin copolymers have preferably been selected from the group consisting of PE (particularly LDPE, LLDPE, HDPE, or mPE), PP, PI, PB, EAA, EMAA, EVA, EPC, and I, and copolymers of these.

“PE” means polyethylene, and “PP” means polypropylene. The polypropylene can be atactic, isotactic, or syndiotactic. “LDPE” means low-density polyethylene, the density of which is in the range from 0.86 to 0.93 g/cm³, and which features a high degree of branching of the molecules. Linear low-density polyethylene (LLDPE) forms a subclass of LDPE and, alongside ethylene as comonomer, contains one or more α-olefins having more than 3 carbon atoms, e.g. 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. Copolymerization of the monomers mentioned results in the molecular structure typical of LLDPE, characterized via a linear main chain on which there are pendant chains. Density varies from 0.86 to 0.94 g/cm³. “HDPE” means high-density polyethylene, which has only very little branching of the molecular chain, possible density here being in the range from 0.94 to 0.97 g/cm³. “mPE” means an ethylene copolymer polymerized by means of metallocene catalysts. An α-olefin having 4 or more carbon atoms is preferably used as comonomer. Density is preferably from 0.88 to 0.93 g/cm³. Polydispersity M_(w)/M_(n) is preferably smaller than 3.5, with preference smaller than 3.0. The melt flow index MFR of polymers based on polyethylene is preferably from 0.3 to 15 g/10 min (at 190° C., load 2.16 kg, measured to DIN EN ISO 1133). The melt flow index MFR of polymers based on polypropylene is preferably from 0.3 to 30 g/10 min (at 230° C., load 2.16 kg, measured to DIN EN ISO 1133).

“PI” means polyisobutylene, and “PB” means polybutylene.

“EAA” means copolymers composed of ethylene and acrylic acid, and “EMAA” means copolymers composed of ethylene and methacrylic acid. In each case, ethylene content is preferably from 60 to 99 mol %.

“EVA” means a copolymer composed of ethylene and vinyl acetate. Ethylene content is preferably from 60 to 99 mol %.

“EPC” means ethylene-propylene copolymers having from 1 to 10 mol % of ethylene, and the ethylene here preferably has random distribution within the molecule.

“I” means copolymers based on olefins and having molecules crosslinked by way of ionic bonds (ionomers). The ionic bonds are reversible, the result of this being that the ionic bonds are broken at conventional processing temperatures (from about 180 to 290° C.) and are formed again during the cooling phase. The polymers usually used are copolymers composed of ethylene with acrylic acids, crosslinked to one another by way of zinc ions, for example, an example being Surlyn®.

In one preferred embodiment of the inventive multilayer foil, all of the polyolefin layers, identical or different, are based on PE, in particular on a PE selected from the group consisting of LDPE, LLDPE, HDPE and mPE.

The oxygen transmission of the inventive multilayer foil is at most 0.5 cm³ m⁻² d⁻¹ bar⁻¹, preferably at most 0.45 or 0.40 cm³ m⁻² d⁻¹ bar⁻¹, more preferably at most 0.35, 0.30, or 0.25 cm³ m⁻² d⁻¹ bar⁻¹, still more preferably at most 0.20, 0.15 or 0.10 cm³ m⁻² d⁻¹ bar⁻¹, most preferably at most 0.09, 0.08, or 0.07 cm³ m⁻² d⁻¹ bar⁻¹, and in particular at most 0.06, 0.05, or 0.04 cm³ m⁻² d⁻¹ bar⁻¹ (in each case at 23° C. and 75% rel. humidity). Suitable methods for determination of oxygen transmission are known to the person skilled in the art. According to the invention, it is preferable that oxygen transmission is determined to DIN ISO 53 380.

In one preferred embodiment of the inventive multilayer foil, after deep-draw thermoforming with a deep-draw thermoforming ratio of 1:3, any amount by which the oxygen transmission in the region subjected to deep-draw thermoforming is smaller than that in the regions not subjected to deep-draw thermoforming is at most 25%, more preferably at most 20%, even more preferably at most 15%, most preferably at most 10%, and in particular at most 5%.

According to the invention, it is preferable that the water-vapor transmission of the inventive multilayer foil likewise amounts at most to the abovementioned values, water-vapor transmission preferably being determined to DIN ISO 53 122.

The inventive multilayer foil has n gas-barrier layers. The person skilled in the art is aware of suitable polymers which are characterized via gas-barrier action. Examples are ethylene-vinyl alcohol copolymers (EVOH), polyvinylidene chloride (PVDC), and vinylidene chloride copolymer, preferably with 80% or greater content of vinylidene chloride, also if appropriate in the form of a blend with other polymers, such as EVA. The gas-barrier layers, respectively identical or different, are preferably based on ethylene-vinyl alcohol copolymer.

The high level of gas-barrier action, in particular the very low oxygen transmission, of the inventive multilayer foil is in particular attributable to the gas-barrier layers. In one preferred embodiment of the inventive multilayer foil, n>1, and the total of the layer thicknesses of the gas-barrier layers is at least 20, 22, or 24 μm, more preferably at least 26, 28, or 30 μm, still more preferably at least 32, 34, or 36 μm, most preferably at least 38, 40, or 42 μm, and in particular at least 44, 46, or 48 μm.

n is particularly preferably 2 or 3, the inventive multilayer foil therefore having two gas-barrier layers B₁ and B₂ and, respectively, three gas-barrier layers B₁, B₂, and B₃, in each case preferably based on EVOH. Surprisingly, it has been found that the thermoform-ability of the inventive multilayer foil can be improved if the total layer thickness of the gas-barrier layers is not combined within an individual layer, but is distributed over a plurality of gas-barrier layers. The total layer thickness of the gas-barrier layers, i.e. the total of all of the layer thicknesses of the individual gas-barrier layers, can have uniform or non-uniform distribution over the individual gas-barrier layers. For a given i, it is preferable that the layer thickness of the gas-barrier layer B_(i) deviates by at most 5 μm from the layer thickness of the gas-barrier layer B_(i+1), more preferably at most 4 μm, still more preferably at most 3 μm, most preferably at most 2 μm, and in particular at most 1 μm.

In one preferred embodiment of the inventive multilayer foil, it has a protective layer S which forms one of the two surface layers of the multilayer foil, where the protective layer S

-   -   is based on a thermoplastic polyolefin, olefin copolymer, or         mixture of these; and/or     -   has a layer thickness of at least 100 μm, more preferably at         least 120 μm, still more preferably at least 140 μm, most         preferably at least 150 μm, and in particular at least 160 μm;         and/or     -   comprises a filler.

Polymers that can be used as polyolefin, olefin copolymer, or mixture of these are in principle the same as those mentioned above in connection with the polyolefin layers. The protective layer S is preferably based on PE, in particular on a PE selected from the group consisting of LDPE, LLDPE, HDPE, and mPE. The protective layer S has the advantage of further reducing the water-vapor transmission of the multilayer foil, providing mechanical stabilization of the gas-barrier layer or gas-barrier layers, and also giving the multilayer foil additional mechanical stability and rigidity after it has been deep-draw thermoformed. It is preferable that the protective layer S is laminated onto the polyolefin layer P₁ or onto the polyolefin layer P_(n+1) and that it forms that surface layer of the multilayer foil intended for subsequent exposure to a relatively high level of external mechanical effects. This lamination process can use either solvent-free lamination methods or else solvent-based lamination methods.

The protective layer S preferably comprises a filler. Suitable fillers are known to the person skilled in the art. The filler is preferably a color pigment, with preference TiO₂

In one preferred embodiment, the inventive multilayer foil has two identical or different protective layers S, which form the two surface layers of the inventive multilayer foil. In this embodiment, it is preferable that at least one of the two protective layers S is heat-sealable. In this case, it is preferable that no filler is present in the heat-sealable protective layer S.

In another preferred embodiment, the polyolefin layer P₁ or the polyolefin layer P_(n+1) forms one surface layer of the multilayer foil. If the multilayer foil has a protective layer S which forms one of the two surface layers of the multilayer foil, the polyolefin layer P₁ or the polyolefin layer P_(n+1) then preferably forms the second of the two surface layers of the multilayer foil.

This second of the two surface layers is preferably heat-sealable. The sealing procedure is described by way of example in Hernandez/Selke/Culter: Plastics Packaging, Carl Hanser Verlag, Munich, 2000.

According to the invention, it is preferable that the heat-sealable polyolefin layer P₁ or P_(n+1) is based on at least one polyolefin selected from the group consisting of mPE, HDPE, LDPE, LLDPE, EVA, EAA, I, (preferably Surlyn® e.g. having zinc ions), PP, preferably homo-PP, and propylene copolymer, or a mixture of these. The sealing temperatures are preferably in the range from 100° C. to 164° C. The melting point is preferably from 90 to 164° C., particularly preferably from 95° C. to 130° C. The heat-sealable polyolefin layer P₁ or P_(n+1) can be equipped with the usual auxiliaries, such as antistatic agents, lubricants, slip agents, antiblocking agents, antifogging agents, and/or spacers.

In one preferred embodiment, the inventive multilayer foil encompasses n intermediate layers Z_(i), where i=from 1 to n, and n intermediate layers Z_(i), where i=from 1 to n, where, for a given i, the gas-barrier layer B_(i), on its side facing toward the polyolefin layer P_(i), directly adjoins the intermediate layer Z_(i), and on its side facing the polyolefin layer P_(i+1) directly adjoins the intermediate layer Z_(i)′. For n=1, the corresponding result is the layer sequence P₁/Z₁/B₁/Z₁′/P₂, and for n=2, the layer sequence P₁/Z₁/B₁/Z₁′/P₂/Z₂/B₂/Z₂′/P₃, and for n=3 the layer sequence is P₁/Z₁/B₁/Z₁′/P₂/Z₂/B₂/Z₂′/P₃/Z₃/B₃/Z₃′/P₄.

The intermediate layers protect the gas-barrier layers, the result being that the thermoforming process does not give rise to any damage or to any attendant drastic reduction in the level of barrier action.

It is preferable that the intermediate layers, respectively identical or different, are based on a polyamide, copolyamide, or mixture of these. For the purposes of the description, polyamides (PA) and copolyamides (CoPA) are preferably aliphatic or (semi)aromatic. The polyamide is preferably aliphatic. The polyamide or copolyamide preferably has a melting point in the range from 160 to 240° C., more preferably from 170 to 220° C. The polyamide or copolyamide has preferably been selected from the group consisting of PA 4, PA 6, PA 7, PA 8, PA 9, PA 10, PA 11, PA 12, PA 4,2, PA 6,6, PA 6,8, PA 6,9, PA 6,10, PA 6,12, PA 7,7, PA 8,8, PA 9,9, PA 10,9, PA 12,12, PA 6/6,6, PA 6,6/6, PA 6,2/6,2 and PA 6,6/6,9/6. PA 6 is particularly preferred. A detailed description of PA and CoPA is found in Kunststoff-Handbuch [Plastics Handbook], Volume VI, Polyamide [Polyamides], Carl Hanser Verlag Munich, 1966; and Melvin I. Kohan, Nylon Plastics Handbook, Carl Hanser Verlag Munich, 1995, the entire content of which is incorporated herein by way of reference.

The layer thicknesses of any intermediate layers that may be present are preferably the same or different, from 1.0 to 25 μm, more preferably from 1.2 to 15 μm, still more preferably from 1.5 to 10 μm, most preferably from 1.8 to 7.5 μm, and in particular from 2.0 to 5.0 μm.

In one preferred embodiment of the inventive multilayer foil, for a given i, the ratio of the layer thickness of the intermediate layer Z_(i) and/or of the intermediate layer Z_(i)′ to the layer thickness of the gas-barrier layer B_(i) is respectively in the range from 3:1 to 1:7, more preferably from 2.5:1 to 1:6, still more preferably from 2:1 to 1:5, most preferably from 1.5:1 to 1:4, and in particular from, 1:1 to 1:3.

If the inventive multilayer foil has the intermediate layers defined above Z_(i) and Z_(i)′, it preferably moreover encompasses n adhesion-promoter layers H_(i), where i=from 1 to n, and n adhesion-promoter layers H_(i)′, where i=from 1 to n, where, for a given i, the adhesion-promoter layer H_(i) has been arranged between the polyolefin layer P_(i) and the intermediate layer Z_(i) and the adhesion-promoter layer H_(i)′ has been arranged between the intermediate layer Z_(i)′ and the polyolefin layer P_(i+1).

Suitable adhesion promoters are known to the person skilled in the art. The adhesion-promoter layers, identical or different, are preferably based on a mixture composed of polyolefins and/or olefin copolymers, preferably selected from the group consisting of PE, such as LDPE or HDPE, PP, maleic anhydride copolymer (MAH copolymer, grafted) and/or ethylene-vinyl acetate copolymer. Preference is given to anhydride-modified PE, acid copolymers of ethylene, acid-modified ethylene-vinyl acetate, acid-modified ethylene-(meth)acrylate, anhydride-modified ethylene-(meth)acrylate, anhydride-modified ethylene-vinyl acetate, acid/acrylate-modified ethylene-vinyl acetate, and a polymer blend comprising at least one of the abovementioned polymers. MAH copolymers are particularly preferred.

The layer thicknesses of any adhesion-promoter layers that may be present are preferably the same or different, from 1.0 to 25 μm, more preferably from 1.2 to 15 μm, still more preferably from 1.5 to 10 μm, most preferably from 1.8 to 7.5 μm, and in particular from 2.0 to 5.0 μm.

In one particularly preferred embodiment, the inventive multilayer foil encompasses one of the following layer sequences:

for n=1:

-   -   P₁/Z₁/B₁/Z₁′/P₂;     -   S/P₁/Z₁/B₁/Z₁′/P₂;     -   P₁/H₁/Z₁/B₁/Z₁′/H₁′/P₂; or     -   S/P₁/H₁/Z₁/B₁/Z₁′/H₁′/P₂;         for n=2:     -   P₁/Z₁/B₁/Z₁′/P₂/Z₂/B₂/Z₂′/P₃;     -   S/P₁/Z₁/B₁/Z₁′/P₂/Z₂/B₂/Z₂′/P₃;     -   P₁/H₁/Z₁/B₁/Z₁′/H₁′/P₂/H₂/Z₂/B₂/Z₂′/H₂′/P₃; or     -   S/P₁/H₁/Z₁/B₁/Z₁′/H₁′/P₂/H₂/Z₂/B₂/Z₂′/H₂′/P₃.         for n=3:     -   P₁/Z₁/B₁/Z₁′/P₂/Z₂/B₂/Z₂′/P₃/Z₃/B₃/Z₃′/P₄;     -   S/P₁/Z₁/B₁/Z₁′/P₂/Z₂/B₂/Z₂′/P₃/Z₃/B₃/Z₃′/P₄;     -   P₁/H₁/Z₁/B₁/Z₁′/H₁′/P₂/H₂/Z₂/B₂/Z₂′/H₂′/P₃/H₃/Z₃/B₃/Z₃′/H₃′/P₄         or     -   S/P₁/H₁/Z₁/B₁/Z₁′/H₁′/P₂/H₂/Z₂/B₂/Z₂′/H₂′/P₃/H₃/Z₃/B₃/Z₃′/H₃′/P₄.

In one preferred embodiment, the inventive multilayer foil has a modulus of elasticity in the tensile test (quotient calculated from the difference in tensile stress and from the corresponding difference in tensile strain at 0.05 and 0.25% tensile strain) of at least 250 MPa, more preferably at least 400 MPa, still more preferably at least 500 MPa, most preferably at least 600 MPa, and in particular at least 650 MPa. Tensile stress at 2% tensile strain is preferably at least 5 MPa, more preferably at least 7.5 MPa, and in particular at least 10 MPa. Tensile stress at 5% tensile strain is preferably at least 8 MPa, more preferably at least 12 MPa, and in particular at least 16 MPa. Tensile stress at break is preferably at least 20 MPa, more preferably at least 30 MPa, and in particular at least 35 MPa. Tensile strain at break is preferably at least 400%, more preferably at least 500%, and in particular at least 550%. The values measured above are preferably determined to DIN EN ISO 527 parts 1+3.

The total layer thickness of the inventive multilayer foil is preferably at least 150 μm, more preferably at least 200 μm, still more preferably at least 250 μm, most preferably at least 300 μm, and in particular at least 350 μm.

The inventive multilayer foil can be printed, and at least one layer of the multilayer foil can be printed here, or can be colored via addition of additives, such as organic or inorganic dyes and pigments.

The inventive multilayer foil is preferably suitable for use at temperatures in the range from −80° C. to +130° C., more preferably from −65° C. to +110° C., and still more preferably from −50° C. to +90° C.

The inventive multilayer foil can be produced with the aid of conventional processes. Production of the inventive multilayer foil can encompass, as constituent step, a flat-foil process, a blowing process, a coating process, a (co)extrusion process, or an appropriate laminating or coating process. It is also possible to combine these processes.

It is preferable that the individual layers of the inventive multilayer foils are joined together neither in a purely sequential process nor in a single step. According to the invention it is preferable that production of the multilayer foils takes place via a consecutive process in which a multilayer composite is first produced, encompassing only a portion of the layers of the inventive multilayer foil. This multilayer composite preferably encompasses only one of the gas-barrier layers.

If, by way of example, n=2, and if the inventive multilayer foil encompasses the layer sequence P₁/Z₁/B₁/Z₁′/P₂/Z₂/B₂/Z₂′/P₃, it is preferable that two multilayer composites P₁/Z₁/B₁/Z₁/P₂ are first produced separately and that these are then joined together. In one particularly preferred embodiment, these two multilayer composites are identical.

The abovementioned multilayer composites are preferably produced via conventional flat-foil coextrusion or via blown-film coextrusion. Processes of this type are known to the person skilled in the art. In this connection reference can be made by way of example to A. L. Brody, K. S. Marsh, The Wiley Encyclopedia of Packaging Technology, Wiley-Interscience, 2nd edition (1997); W. Soroka, Fundamentals of Packaging Technology, Institute of Packaging Professionals (1995); J. Nentwig, Kunststoff-Folien [Plastics Foils], Hanser Fachbuch (2000); and S. E. M. Selke, Understanding Plastics Packaging Technology (Hanser Understanding Books), Hanser Gardner Publications (1997).

The two composites are preferably joined together immediately after the coextrusion process, i.e. while heat is retained. If the surface layers joined together of the two composites are based on the same (co)polymer or on a chemically related (co)polymer, the residual heat from the coextrusion process is usually sufficient to achieve sufficient adhesion.

When two identical multilayer composites of P₁/Z₁/B₁/Z₁′/P₂ type are joined together, the result is a P₁/Z₁/B₁/Z₁′/(P₂—P₂)/Z₂/B₂/Z₂′/P₃ composite. Since the surface layers joined together are based on the same polymers, the joining process produces a unitary layer, shown as “(P₂—P₂)” in the above composite. If, in contrast, the layers joined together are based on different polymers, the resultant composite can be written as P₁/Z₁/B₁/Z₁′/P₂/X/Z₂/B₂/Z₂′/P₃ in compliance with the inventive definition (indication of individual layers), where P₂ is the polyolefin layer joined together from one of the multilayer composites and X is the polyolefin layer joined together from the other multilayer composite.

However, in principle it is also possible to use other methods to join the multilayer composites together, for example by using an extrusion-lamination process to extrude a tie layer between the two multilayer composites.

Another aspect of the invention provides a packaging tray encompassing a multilayer foil described above, where this foil has been thermoformed, preferably deep-draw thermoformed. The inventive packaging tray is preferably one whose oxygen transmission, not only in the thermoformed regions but also in the non-thermoformed regions, is at most 0.5 cm³ m⁻² d⁻¹ bar⁻¹, preferably at most 0.45 or 0.40 cm³ m² d⁻¹ bar⁻¹, more preferably at most 0.35, 0.30, or 0.25 cm³ m⁻² d⁻¹ bar⁻¹, still more preferably at most 0.20, 0.15, or 0.10 cm³ m⁻² d⁻¹ bar⁻¹, most preferably at most 0.09, 0.08, or 0.07 cm³ m⁻² d⁻¹ bar⁻¹, and in particular at most 0.06, 0.05 or 0.04 cm³ m⁻² d⁻¹ bar⁻¹ (in each case at 23° C. and 75% rel. humidity). The person skilled in the art is aware of suitable methods for determination of oxygen transmission. According to the invention, it is preferable that oxygen transmission is determined to DIN ISO 53 380.

The person skilled in the art is aware of suitable devices for the thermoforming process, for example for the deep-draw thermoforming process.

Another aspect of the invention provides the use of the inventive packaging tray for the packaging of goods.

Another aspect of the invention provides a process for production of a packaging tray composed of the inventive, thermoformable multilayer foil described above. The inventive process encompasses the thermoforming step, preferably the step of deep-draw thermoforming of the multilayer foil. The invention also provides thermoformed packaging trays which are obtainable by this process, and the use of the thermoformable multilayer foil for production of a packaging tray.

Another aspect of the invention provides a packaging encompassing the inventive, thermoformed packaging tray described above.

In one preferred embodiment, the inventive packaging tray has been sealed by a lid foil, and the interior space formed by packaging tray and lid foil in the packaging is at subatmospheric pressure. The pressure in the interior space is preferably less than 20 mbar, more preferably less than 15 mbar, still more preferably less than 10 mbar, and most preferably less than 5 mbar.

In one preferred embodiment, this pressure is maintained for a period of at least 12 months, more preferably at least 18 months, and in particular at least 24 months, i.e. the pressure change is less than 5% in this period.

The lid foil has preferably been metallized. Suitable methods for metallizing of plastics foils are known to the person skilled in the art. In the course of the metallizing process, only a comparatively thin metal film is applied to the layer, the thickness of the film preferably being in the range from 5 to 100 nm, more preferably from 10 to 50 nm. Plasma coating can be used to produce metallized foils of this type.

Various metals or mixtures of these are suitable for the metallizing process. According to the invention, the lid foil is preferably a multilayer foil and at least one layer has been metallized with aluminum or with copper, particularly preferably aluminum.

In one preferred embodiment, the lid foil encompasses a plurality of layers of which at least one is based on a polyester, copolyester, or a mixture of these.

For the purposes of the description, polyesters and copolyesters are preferably those selected from the group consisting of PET (in particular cPET or aPET), coPET, PBT and coPBT. “PET” means polyethylene terephthalate, which can be prepared from ethylene glycol and terephthalic acid. A distinction can moreover be drawn between amorphous PET (aPET) and crystalline PET (cPET). “coPET” means copolyesters which contain not only ethylene glycol and terephthalic acid but also other monomers, e.g. branched or aromatic diol glycols. “PBT” means polybutylene terephthalate, and “coPBT” means a copolyester of polybutylene terephthalate. PBT can be prepared from 1,4-butanediol and terephthalic acid. The intrinsic viscosity of the polyester or copolyester is preferably from 0.1 to 2.0 dl/g, more preferably from 0.2 to 1.7 dl/g, still more preferably from 0.3 to 1.5 dl/g, most preferably from 0.4 to 1.2 dl/g, and in particular from 0.6 to 1.0 dl/g. The person skilled in the art is aware of methods for intrinsic viscosity determination. A detailed description of PET, PBT, polycarbonates (PC), and copolycarbonates (coPC) is found in Kunststoffhandbuch [Plastics handbook], volume 3/1-technische Thermoplaste: Polycarbonate, Polyacetale, Polyester, Celluloseester [Industrial thermoplastics: polycarbonates, polyacetals, polyesters, cellulose esters]; Carl Hanser Verlag, 1992, the entire content of which is incorporated by way of reference.

The lid foil particularly preferably encompasses four layers, of which three layers have been metallized. The layers may have been bonded to one another with the aid of suitable laminating adhesives. It is preferable here that two layers are based on a polyester or copolyester and that the other two layers are based on a polyolefin or olefin copolymer. It is particularly preferable that the lid foil encompasses a heat-sealable layer based on PE, a metallized layer based on PET, a metallized layer based on biaxially oriented PP (BOPP), and a metallized layer based on PET, preferably in this layer sequence. The arrangement of the metallized surfaces (m) within the lid foil preferably corresponds to the following schematic representation: PE/PET-m/BOPP-m/m-PET, i.e. the two layers following the PE layer and based on PET on BOPP have preferably been metallized on their side facing away from the PE layer, and the other layer based on PET has preferably been metallized on its side facing toward the PE layer.

In one particularly preferred embodiment, the puncture resistance of the lid foil determined to ASTM F1306 is at least 30 N, more preferably at least 35 N, still more preferably at least 40 N, most preferably at least 45 N, and in particular at least 50 N. The fracture deformation here is preferably at least 2 mm, more preferably at least 5 mm, and in particular at least 8 mm.

The inventive packaging tray has preferably been sealed to the lid foil. To this end, it is preferable that the heat-sealable layer of the packaging tray and the heat-sealable layer of the lid foil are based on the same polymer or on an at least chemically related polymer. The lid foil can be heat-sealed to the packaging tray with the aid of conventional sealing devices at the usual temperatures.

Another aspect of the invention provides a transport container encompassing the inventive packaging described above. It is preferable that the inventive transport container has been equipped with cooling equipment, of which the inventive packaging is a constituent. The cooling principle is preferably based on the evaporation of a liquid at reduced pressure. It is preferable that the transport container has external thermal insulation and has a cavity into which it is possible to introduce goods for packaging which are to be transported and which are preferably heat-sensitive. This particularly preferably involves the type of self-cooling transport container described by way of example in U.S. Pat. No. 6,584,797, U.S. Pat. No. 6,688,132 or U.S. Pat. No. 6,701,724.

The examples below serve for further explanation of the invention, but are not to be interpreted as restricting its scope:

EXAMPLE 1

Coextrusion was used to produce a multilayer composite of the following structure: PE1/H/PA/EVOH/PA/H/PE2. Directly after the coextrusion process, the multilayer composite was joined together on one side with a multilayer composite of the same type, thus producing a multilayer foil of the following structure:

PE1/H/PA/EVOH/PA/H/PE2/(EVA-EVA)/PE2/H/PA/EVOH/PA/H/PE1

PE1: 35 μm

PE2: 20 μm

PA: 10 μm

EVOH: 15 μm

H: 5 μM

EVA: 2.5 μm

EXAMPLE 2

By analogy with example 1, a multilayer composite was produced, but was not joined together with a second multilayer composite of the same type. The result was no division of the functional barrier layer (EVOH) into a plurality of layers, and the multilayer composite itself corresponded to the finished multilayer foil of the following layer structure:

PE/H/PA/EVOH/PA/H/PE

PE: 60 μm

H: 5 μm

PA: 20 μm

EVOH: 30 μm

EXAMPLE 3

The multilayer foil from example 1 was laminated onto an additional PE foil (PE3). This gave a multilayer foil of the following structure:

PE1/H/PA/EVOH/PA/H/PE2/(EVA-EVA)/PE2/H/PA/EVOH/PA/H/PE1/PE3

PE3: 150 μm

The additional PE foil was laminated onto the composite from example 1 in order to lower the water-vapor barrier. This firstly gave a further mechanical protective layer for the EVOH via this PE layer (thickness at least 150 μm), and secondly gave additional stability to the foil after deep-draw thermoforming. The PE layer is preferably laminated onto the side intended for a relatively high level of exposure to external mechanical forces.

EXAMPLE 4

The multilayer foil of example 1 was laminated on both sides, in each case to an additional PE layer (PE4). This gave a multilayer foil of the following structure:

PE4/PE1/H/PA/EVOH/PA/H/PE2/(EVA-EVA)/PE 2/H/PA/EVOH/PA/H/PE1/PE4

PE4: 75 μm

The two additional PE layers firstly lower the water-vapor transmission and secondly also lower the ability to resist mechanical effects either from the inside or from the outside. 

1. A thermoformable multilayer foil encompassing at least one polyolefin layer based on a thermoplastic polyolefin, olefin copolymer, or mixture of these, and at least one gas-barrier layer, where the oxygen transmission of the multilayer foil is at most 0.5 cm³ m⁻² d⁻¹ bar⁻¹.
 2. The multilayer foil as claimed in claim 1, which encompasses n gas-barrier layers B_(i), where i=from 1 to n; and n+1 polyolefin layers P_(i), where i=from 1 to n+1, where n=1, 2, 3, 4, or 5; the polyolefin layers, respectively identical or different, are based on a thermoplastic polyolefin, olefin copolymer, or mixture of these; and for a given i, the gas-barrier layer B_(i) has been arranged between the polyolefin layers P_(i) and P_(i+1).
 3. The multilayer foil as claimed in claim 1 or 2, wherein the gas-barrier layer or gas-barrier layers, respectively identical or different, are based on ethylene-vinyl alcohol copolymer.
 4. The multilayer foil as claimed in claim 2 or 3, wherein n>1, where the total of the layer thicknesses of the gas-barrier layers is at least 20 μm; and/or for a given i, the layer thickness of the gas-barrier layer B_(i) deviates by at most 5 μm from the layer thickness of the gas-barrier layer B_(i+1).
 5. The multilayer foil as claimed in any of claims 2 to 4, which has a protective layer S which forms at least one of the two surface layers of the multilayer foil, where the protective layer S is based on a thermoplastic polyolefin, olefin copolymer, or mixture of these; and/or has a layer thickness of at least 100 μm; and/or comprises a filler.
 6. The multilayer foil as claimed in any of claims 2 to 5, wherein the polyolefin layer P₁ or the polyolefin layer P_(n+1) forms a surface layer of the multilayer foil and, if appropriate, is heat-sealable.
 7. The multilayer foil as claimed in any of claims 2 to 6, which encompasses n intermediate layers Z_(i), where i=from 1 to n, and n intermediate layers Z_(i)′, where i=from 1 to n, where, for a given i, the gas-barrier layer B_(i), on its side facing toward the polyolefin layer P_(i), directly adjoins the intermediate layer Z_(i), and on its side facing the polyolefin layer P_(i+1) directly adjoins the intermediate layer Z_(i)′.
 8. The multilayer foil as claimed in claim 7, wherein the intermediate layers, respectively identical or different, are based on a polyamide, copolyamide, or mixture of these.
 9. The multilayer foil as claimed in claim 7 or 8, wherein, for a given i, the ratio of the layer thickness of the intermediate layer Z_(i) and/or of the intermediate layer Z_(i)′ to the layer thickness of the gas-barrier layer B_(i) is respectively in the range from 3:1 to 1:7.
 10. The multilayer foil as claimed in any of claims 7 to 9, which encompasses n adhesion-promoter layers H_(i), where i=from 1 to n, and n adhesion-promoter layers H_(i)′, where i=from 1 to n, where, for a given i, the adhesion-promoter layer H_(i) has been arranged between the polyolefin layer P_(i) and the intermediate layer Z_(i) and the adhesion-promoter layer H_(i)′ has been arranged between the intermediate layer Z_(i)′ and the polyolefin layer P_(i+1).
 11. The multilayer foil as claimed in any of the preceding claims, wherein, after deep-draw thermoforming with a deep-draw thermoforming ratio of 1:3, any amount by which the oxygen transmission in the region subjected to deep-draw thermoforming is smaller than that in the regions not subjected to deep-draw thermoforming is at most 25%.
 12. The multilayer foil as claimed in any of the preceding claims, whose total layer thickness is at least 150 μm.
 13. A packaging tray encompassing a thermoformed multilayer foil as claimed in any of claims 1 to
 12. 14. The packaging tray as claimed in claim 13, which has an oxygen transmission of at most 0.5 cm³ m⁻² d⁻¹ bar⁻¹ not only in the thermoformed regions but also in the non-thermoformed regions.
 15. A packaging encompassing a packaging tray as claimed in claim 13 or
 14. 16. The packaging as claimed in claim 15, wherein the packaging tray has been sealed by a lid foil, and the interior space formed by packaging tray and lid foil in the packaging is at subatmospheric pressure.
 17. The packaging as claimed in claim 16, wherein the lid foil has been metallized.
 18. The packaging as claimed in claim 16 or 17, wherein the lid foil encompasses a plurality of layers of which at least one is based on a polyester, copolyester, or mixture of these.
 19. A transport container encompassing a packaging as claimed in any of claims 15 to
 18. 